1. Field of the Invention
The present invention relates to a wholly aromatic polyamide fiber synthetic paper sheet. More particularly, the present invention relates to a wholly aromatic polyamide fiber synthetic paper sheet having excellent heat resistance and electric insulating property at a high humidity and thus being useful for preparing a prepreg and a laminate plate for electric circuit.
2. Description of the Related Art
It is well known that, for substrate materials usable for electric circuit laminate plates, high heat resistance, heat-resistant dimensional stability, humidity resistant-dimensional stability, electric insulating property, resistance to deformation, (for example, distortion, curling and corrugation) and light weight are required. Wholly aromatic polyamide fiber synthetic paper sheets are excellent in heat resistance, electric insulating property, heat resistant dimensional stability and are lightweight in comparison with other synthetic paper sheets, and thus are currently utilized in the field of the substrate materials for electric circuit laminate plates.
For example, (1) Japanese Unexamined Patent Publications No. 2-236,907 and No. 2-106,840 disclose synthetic paper sheets formed from poly-m-phenylene isophthalamide staple fibers (trademark: CORNEX made by TEIJIN LIMITED) and poly-m-phenylene isophthalamide pulp fibers (fibrids); (2) Japanese Unexamined Patent Publication No. 1-92,233 discloses a wholly aromatic polyamide fiber synthetic paper sheet formed from poly-p-phenylene terephthalamide staple fibers (trademark: Kevler, made by DU PONT) or copoly-p-phenylene/3,4xe2x80x2-oxydiphenylene terephthalamide staple fibers (trademark: TECHNOLA, made by TEIJIN LTD.) and an organic resin binder; and (3) Japanese Unexamined Patent Publication No. 2-47,392 discloses a process for producing a wholly aromatic polyamide fiber synthetic paper sheet.
The above-mentioned wholly aromatic polyamide fiber synthetic paper sheet (1) is advantageous in high heat resistance. However, the synthetic paper sheet (1) is disadvantageous in that not only when the sheet (1) is heat treated at a high temperature of 250xc2x0 C. or more, the sheet (1) shrinks to change the dimensions thereof, but also in that, since the aromatic polyamide fibers have a high equilibrium moisture content (water content) and a high content of impurity ions, when the synthetic paper sheet (1) is held at a high humidity for a long period, the sheet (1) exhibits a poor electric insulating property, and thus cannot be used for the insulating substrate material. Also, the synthetic paper sheet (2) is advantageous in that the equilibrium moisture content and impurity ion content of the sheet (2) are low. However, the synthetic paper sheet (2) is disadvantageous in that, since the sheet (2) contains only the organic resin as a binder component and in the production procedure of the synthetic paper sheet (2), the binder component migrates to the front and back surface sides of the sheet (2) and is locally contained in the sheet (2), the content of the binder component in the middle portion of the sheet (2) becomes small. Thus, the uniformity of the synthetic paper sheet (2) in the direction of thickness thereof is low and the reliability of the sheet (2) is reduced.
As mentioned above, when the synthetic paper sheets (2) are used as a substrate material for producing the electric circuit laminate plate, in the production procedures for the substrate material, particularly prepreg preparation procedures in which the synthetic paper sheets (2) are impregnated with a mixed varnish containing, for example, an epoxy resin, and dried, and a laminate-shaping procedure in which the prepregs are laminated and shaped, the variability of the distribution of the amount of the mixed varnish impregnated in or adhered to the synthetic paper sheets (2) (especially in the direction of the thickness of the sheets 2) is increased; that a portion of the binder resin is melted so as to cause the adhesion between the fibers to decrease and the synthetic paper sheets to be broken; the staple fibers in the synthetic paper sheets are made easily movable in relation to each other, and thus the uniformity in the distribution of the staple fibers in the synthetic paper sheets is decreased; and, particularly, a deformation of the electric circuit laminate plate is created after a solder-reflow procedure is completed. Therefore, the synthetic paper sheets (2) are not preferred for the electric circuit laminate plate.
To solve the above-mentioned problems, Japanese Unexamined Patent Publication No. 61-160,500 discloses a high density para-aramid fiber synthetic paper sheet in which, as a binder component, meta-type wholly aromatic polyamide fibrids are used in place of the organic resin, and a blend of para-type wholly aromatic polyamide staple fibers (for example, KEVLER (trademark) made by DU PONT) with the finely fibrillated para-type wholly aromatic polyamide fibers (for example, KEVLER, made by DU PONT) is used in combination with the binder component fibrids. The synthetic paper sheet exhibits excellent heat resistance, heat resistant dimensional stability, humidity resistant dimensional stability and resistance to deformation, for example, distortion, curling and corrugation. However, the synthetic paper sheet is disadvantageous in that, in the structure of the synthetic paper sheet, the finely fibrillated para-type wholly aromatic polyamide fibers fill the spaces formed between the para-type wholly aromatic polyamide staple fibers and they are bonded to each other with the fibrids, and therefore, when the synthetic paper sheet is used as a substrate material for the electric circuit laminate plate, the synthetic paper sheet exhibits a reduced impregnation property for the mixed varnish containing, for example, an epoxy resin, and thus the synthetic paper sheet is sometimes unevenly and/or insufficiently impregnated with the mixed varnish, and the resultant electric circuit laminate plate often exhibits an insufficient electric insulating property. Therefore, there is a strong demand to solve the above-mentioned problems of the synthetic paper sheet.
An object of the present invention is to provide a wholly aromatic polyamide fiber synthetic paper sheet having excellent heat resistance and electric insulating properties and useful for a substrate material of an electric circuit laminate plate.
Another object of the present invention is to provide wholly aromatic polyamide fiber synthetic paper sheet which is useful as a substrate material of an electric circuit laminate plate having a high resistance to deformations (for example, distortion, curling and corrugation) during the production thereof, and a high electric insulating property even at a high humidity.
The above-mentioned objects can be attained by the wholly aromatic polyamide fiber synthetic paper sheet of the present invention, which comprises 70 to 96% by weight of a wholly aromatic polyamide staple fiber component and 4 to 30% by weight of a binder component comprising at least one member selected from the group consisting of organic resinous binders and heat-resistant organic polymer fibrids, the wholly aromatic polyamide staple fiber component including 30% by weight or more of para-type wholly aromatic polyamide staple fibers each having at least two annular projections expanding from the peripheral surface of the staple fiber, spaced from each other, and having an average ratio R/xcex3 of the largest diameter R of the annular projections to the smallest diameter xcex3 of the annular projection-free portions of the staple fiber, of 1.1 or more.
Preferably, in the wholly aromatic polyamide fiber synthetic paper sheet of the present invention, the wholly aromatic polyamide staple fibers have a thermal decomposition-starting temperature of 310xc2x0 C. or more, a tensile strength of 16.67 dtex (15 g/denier) and an ultimate elongation of 8% or less.
In the wholly aromatic polyamide staple fiber synthetic paper sheet of the present invention, the wholly aromatic polyamide for the wholly aromatic polyamide staple fibers preferably contain at least 80 molar % of recurring divalent aromatic amide units of the general formula (I):
xe2x80x94NHxe2x80x94Ar1xe2x80x94NHCOxe2x80x94Ar2xe2x80x94COxe2x80x94xe2x80x83xe2x80x83(I)
wherein Ar1 and Ar2 respectingly and independently from each other represent a divalent aromatic group which may have at least one substituent selected from the group consisting of halogen atoms, lower alkyl groups and a phenyl group.
In the formula (I), the aromatic groups Ar1 and Ar2 respectively and independently from each other represent a member selected from the divalent aromatic groups of the formulae: 
which may have at least one substituent selected from the group consisting of halogen atoms, lower alkyl groups and a phenyl group.
Preferably, in the wholly aromatic polyamide staple fiber synthetic paper sheet of the present invention, the wholly aromatic polyamide for the wholly aromatic polyamide staple fibers is a co-polymerization product of an acid component consisting of terephthalic acid with a diamine compound consisting of p-phenylenediamine and 3,4xe2x80x2-oxydiphenylenediamine in a molar ratio of 1:3 to 3:1.
In the wholly aromatic polyamide staple fiber synthetic paper sheet of the present invention, the organic resinous binder preferably comprises at least one thermosetting resist selected from epoxy resins, phenolic compound-formaldehyde resins, polyurethane resins and melamine-formaldehyde resins.
In the wholly aromatic polyamide staple fiber synthetic paper sheet of the present invention, the organic resinous binder preferably comprises a thermosetting organic resin having functional epoxy groups and dispersible in water.
In the wholly aromatic polyamide fiber synthetic paper sheet of the present invention, the wholly aromatic polyamide staple fibers preferably have at least two peaks in a histogram of fiber length frequency distribution thereof.
In the fiber length distribution histogram as mentioned above, the difference in staple fiber length between the fibers distributed in the two or more fiber length frequency distribution columns at peaks is preferably at least 1 mm.
The prepreg of the present invention comprises at least one wholly aromatic polyamide fiber synthetic paper sheet of the present invention and a thermosetting resin with which the wholly aromatic polyamide fiber synthetic paper sheet is impregnated.
The laminate plate of the present invention comprises at least one of the prepregs mentioned above heat-shaped under pressure.